Brainstorming

A method of generating ideas within a group that was originated in the
1940s by Boston advertising Executive Alex Osborn.

Although everyone has 'done it', surprisingly few know the four main
rules:

No judgment or criticism

Quantity over quality

Freewheel

Mutate and combine

Brainstorming is useful but relatively dated method and
there are many ways of sprucing it up.

There are a number vocal of critics of Brainstorming,
including Edward de Bono, who have concluded that individual thinking is
just as good, if not better. The trick is to learn a whole raft of
techniques that you can use by yourself.